Flexible B12 ecophysiology of Phaeocystis antarctica due to a fusion B12-independent methionine synthase with widespread homologues.

Coastal Antarctic marine ecosystems are significant in carbon cycling because of their intense seasonal phytoplankton blooms. Southern Ocean algae are primarily limited by light and iron (Fe) and can be co-limited by cobalamin (vitamin B12). Micronutrient limitation controls productivity and shapes the composition of blooms which are typically dominated by either diatoms or the haptophyte Phaeocystis antarctica. However, the vitamin requirements and ecophysiology of the keystone species P. antarctica remain poorly characterized. Using cultures, physiological analysis, and comparative omics, we examined the response of P. antarctica to a matrix of Fe-B12 conditions. We show that P. antarctica is not auxotrophic for B12, as previously suggested, and identify mechanisms underlying its B12 response in cultures of predominantly solitary and colonial cells. A combination of proteomics and proteogenomics reveals a B12-independent methionine synthase fusion protein (MetE-fusion) that is expressed under vitamin limitation and interreplaced with the B12-dependent isoform under replete conditions. Database searches return homologues of the MetE-fusion protein in multiple Phaeocystis species and in a wide range of marine microbes, including other photosynthetic eukaryotes with polymorphic life cycles as well as bacterioplankton. Furthermore, we find MetE-fusion homologues expressed in metaproteomic and metatranscriptomic field samples in polar and more geographically widespread regions. As climate change impacts micronutrient availability in the coastal Southern Ocean, our finding that P. antarctica has a flexible B12 metabolism has implications for its relative fitness compared to B12-auxotrophic diatoms and for the detection of B12-stress in a more diverse set of marine microbes.

The interacting effect of prolonged darkness and temperature on photophysiological characteristics of three Antarctic phytoplankton species.

Photophysiological characteristics of the Southern Ocean phytoplankton species Phaeocystis antarctica, Geminigera cryophila, and Chaetoceros simplex were assessed during 7 weeks of darkness and subsequent recovery after darkness at 4 and 7°C. Chlorophyll a fluorescence and maximum quantum efficiency of PSII decreased during long darkness in a species-specific manner, whereas chlorophyll a concentration remained mostly unchanged. Phaeocystis antarctica showed the strongest decline in photosynthetic fitness during darkness, which coincided with a reduced capacity to recover after darkness, suggesting a loss of functional photosystem II (PSII) reaction centers. The diatom C. simplex at 4°C showed the strongest capacity to resume photosynthesis and active growth during 7 weeks of darkness. In all species, the maintenance of photosynthetic fitness during darkness was clearly temperature dependent as shown by the stronger decline of photosynthetic fitness at 7°C compared to 4°C. Although we lack direct evidence for this, we suggest that temperature-enhanced respiration rates cause stronger depletion of energy reserves that subsequently interferes with the maintenance of photosynthetic fitness during long darkness. Therefore, the current low temperatures in the coastal Southern Ocean may aid the maintenance of photosynthetic fitness during the austral winter. Further experiments should examine to what extent the species-specific differences in dark survival are relevant for future temperature scenarios for the coastal Southern Ocean.

Phytoplankton growth rates in the Amundsen Sea (Antarctica) during summer: The role of light.

In the Amundsen Sea, significant global warming accelerates ice melt, and is consequently altering many ocean properties such as sea ice concentration, surface freshening, water column stratification, and underwater light properties. To examine the influence of light, which is one of the fundamental factors for phytoplankton growth, incubation experiments and field surveys were performed during the austral summer of 2016. In the incubation experiments, phytoplankton abundance and carbon biomass significantly increased with increasing light levels, probably indicating light limitation. Growth rates of the small pennates (mean 0.42 d-1) increased most rapidly with an increase in light, followed by those of Phaeocystis antarctica (0.31 d-1), and the large diatoms (0.16 d-1). A short-term study during the field survey showed that phytoplankton distribution in the surface layer was likely controlled by different responses to light and the sinking rate of each species. These results suggest that the approach adopted by previous studies of explaining phytoplankton ecology as a characteristic of two major taxa, namely diatoms and P. antarctica, in the coastal Antarctic waters might cause errors owing to oversimplification and misunderstanding, since diatoms comprise several species that have different ecophysiological characteristics.

Ice Melting Can Change DMSP Production and Photosynthetic Activity of the Haptophyte Phaeocystis antarctica1.

Phaeocystis antarctica is an important primary producer in the Southern Ocean and plays roles in sulfur cycles through intracellular production of dimethylsulfoniopropionate (DMSP), a principal precursor of dimethyl sulfide (DMS). Haptophytes, including P. antarctica, are known to produce more DMSP than other phytoplankton groups such as diatoms and green algae, suggesting their important contribution to DMS concentrations in the Southern Ocean. We assessed how sea ice formation and melting affect photosynthesis and DMSP accumulation in P. antarctica both in seawater and in sea ice. Incubations were undertaken in an ice tank, which simulated sea ice formation and melting dynamics. The maximum quantum yield of photochemistry (Fv /Fm ) in photosystem II, as estimated from pulse-amplitude-modulated (PAM) fluorometry, was generally higher under low-light conditions than high-light conditions. Values of Fv /Fm , the relative maximum electron rate (rETRmax ), and photosynthetic efficiency (α) were lower in sea ice than in seawater, implying reduced photosynthetic function inside the sea ice. The reduction in photosynthetic function was probably due to the hypersaline environment in the brine channels. Total DMSP (DMSPt) concentration normalized by chlorophyll-a concentration was significantly higher in the sea ice than in the other environments, suggesting high accumulation of DMSP, probably due to its osmotic properties. Fv /Fm , specific growth rate, and DMSPt concentrations decreased with decreasing salinity with the lowest values found at a salinity of 22, that is, the lowest salinity tested. These results suggest that sea ice melting is responsible for a reduction in growth rate and DMSP production of P. antarctica.

Light-adaptive state transitions in the Ross Sea haptophyte Phaeocystis antarctica and in dinoflagellate cells hosting kleptoplasts derived from it.

Light state transitions (STs) is a reversible physiological process that oxygenic photosynthetic organisms use in order to minimize imbalances in the electronic excitation delivery to the reaction centers of Photosystems I and II, and thus to optimize photosynthesis. STs have been studied extensively in plants, green algae, red algae and cyanobacteria, but sparsely in algae with secondary red algal plastids, such as diatoms and haptophytes, despite their immense ecological significance. In the present work, we examine whether the haptophyte alga Phaeocystis antarctica, and dinoflagellate cells that host kleptoplasts derived from P. antarctica, both endemic in the Ross Sea, Antarctica, are capable of light adaptive STs. In these organisms, Chl a fluorescence can be excited either by direct light absorption, or indirectly by electronic excitation (EE) transfer from ultraviolet light absorbing mycosporine-like amino acids (MAAs) to Chl a (Stamatakis et al., Biochim. Biophys. Acta 1858 [2017] 189-195). Here we show that, on adaptation to PS II-selective light, dark-adapted P. antarctica cells shift from light state 1 (ST1; more EE ending up in PS II) to light state 2 (ST2; more EE ending up in PS I), as revealed by the spectral distribution of directly-excited Chl a fluorescence and by changes in the macro-organization of pigment-protein complexes evidenced by circular dichroism (CD) spectroscopy. In contrast, no STs are clearly detected in the case of the kleptoplast-hosting dinoflagellate cells, and in the case of indirectly excited Chls a, via MAAs, in P. antarctica cells.

The extraordinary longevity of kleptoplasts derived from the Ross Sea haptophyte Phaeocystis antarctica within dinoflagellate host cells relates to the diminished role of the oxygen-evolving Photosystem II and to supplementary light harvesting by mycosporine-like amino acid/s.

The haptophyte Phaeocystis antarctica and the novel Ross Sea dinoflagellate that hosts kleptoplasts derived from P. antarctica (RSD; R.J. Gast et al., 2006, J. Phycol. 42 233-242) were compared for photosynthetic light harvesting and for oxygen evolution activity. Both chloroplasts and kleptoplasts emit chlorophyll a (Chl a) fluorescence peaking at 683nm (F683) at 277K and at 689 (F689) at 77K. Second derivative analysis of the F689 band at 77K revealed two individual contributions centered at 683nm (Fi-683) and at 689 (Fi-689). Using the p-nitrothiophenol (p-NTP) treatment of Kobayashi et al. (Biochim. Biophys. Acta 423 (1976) 80-90) to differentiate between Photosystem (PS) II and I fluorescence emissions, we could identify PS II as the origin of Fi-683 and PS I as the origin of Fi-689. Both emissions could be excited not only by Chl a-selective light (436nm) but also by mycosporine-like amino acids (MAAs)-selective light (345nm). This suggests that a fraction of MAAs must be proximal to Chls a and, therefore, located within the plastids. On the basis of second derivative fluorescence spectra at 77K, of p-NTP resolved fluorescence spectra, as well as of PSII-driven oxygen evolution activities, PS II appears substantially less active (~1/5) in dinoflagellate kleptoplasts than in P. antarctica chloroplasts. We suggest that a diminished role of PS II, a known source of reactive oxygen species, and a diminished dependence on nucleus-encoded light-harvesting proteins, due to supplementary light-harvesting by MAAs, may account for the extraordinary longevity of RSD kleptoplasts.

Selective feeding and foreign plastid retention in an Antarctic dinoflagellate.

The peridinin-containing plastid found in most photosynthetic dinoflagellates is thought to have been replaced in a few lineages by plastids of chlorophyte, diatom, or haptophyte origin. Other distinct lineages of phagotrophic dinoflagellates retain functional plastids obtained from algal prey for different durations and with varying source species specificity. 18S rRNA gene sequence analyses have placed a novel gymnodinoid dinoflagellate isolated from the Ross Sea (RSD) in the Kareniaceae, a family of dinoflagellates with permanent plastids of haptophyte origin. In contrast to other species in this family, the RSD contains kleptoplastids sequestered from its prey, Phaeocystis antarctica. Culture experiments were employed to determine whether the RSD fed selectively on P. antarctica when offered in combination with another polar haptophyte or cryptophyte species, and whether the RSD, isolated from its prey and starved, would take up plastids from P. antarctica or from other polar haptophyte or cryptophyte species. Evidence was obtained for selective feeding on P. antarctica, plastid uptake from P. antarctica, and increased RSD growth in the presence of P. antarctica. The presence of a peduncle-like structure in the RSD suggests that kleptoplasts are obtained by myzocytosis. RSD cells incubated without P. antarctica were capable of survival for at least 29.5 months. This remarkable longevity of the RSD's kleptoplasts and its species specificity for prey and plastid source is consistent with its prolonged co-evolution with P. antarctica. It may also reflect the presence of a plastid protein import mechanism and genes transferred to the dinokaryon from a lost permanent haptophyte plastid.

Phaeocystis antarctica blooms strongly influence bacterial community structures in the Amundsen Sea polynya.

Rising temperatures and changing winds drive the expansion of the highly productive polynyas (open water areas surrounded by sea ice) abutting the Antarctic continent. Phytoplankton blooms in polynyas are often dominated by the haptophyte Phaeocystis antarctica, and they generate the organic carbon that enters the resident microbial food web. Yet, little is known about how Phaeocystis blooms shape bacterial community structures and carbon fluxes in these systems. We identified the bacterial communities that accompanied a Phaeocystis bloom in the Amundsen Sea polynya during the austral summers of 2007-2008 and 2010-2011. These communities are distinct from those determined for the Antarctic Circumpolar Current (ACC) and off the Palmer Peninsula. Diversity patterns for most microbial taxa in the Amundsen Sea depended on location (e.g., waters abutting the pack ice near the shelf break and at the edge of the Dotson glacier) and depth, reflecting different niche adaptations within the confines of this isolated ecosystem. Inside the polynya, P. antarctica coexisted with the bacterial taxa Polaribacter sensu lato, a cryptic Oceanospirillum, SAR92 and Pelagibacter. These taxa were dominated by a single oligotype (genotypes partitioned by Shannon entropy analysis) and together contributed up to 73% of the bacterial community. Size fractionation of the bacterial community [<3 µm (free-living bacteria) vs. >3 µm (particle-associated bacteria)] identified several taxa (especially SAR92) that were preferentially associated with Phaeocystis colonies, indicative of a distinct role in Phaeocystis bloom ecology. In contrast, particle-associated bacteria at 250 m depth were enriched in Colwellia and members of the Cryomorphaceae suggesting that they play important roles in the decay of Phaeocystis blooms.